Technique and apparatus for radioactive source preparation



L. G. LEWIS Aug. 28, 1956 TECHNIQUE AND APPARATUS FOR RADIOACTIVE SOURCEPREPARATION Filed July 29, 1953 m 7 EEEEEE a I i 3/0 35 INVENTOR.

Lloyd 6. Lewis ATTORNEY United States Patent TECHNIQUE AND APPARATUS FORRADIQ- ACTIVE SQURCE PREPARATION Lloyd G. Lewis, La Grange, illlL,assignor to Standard Oil Company, Chicago, 111., a corporation ofIndiana Application July 29, 1953, Serial No. 371,137

9 Claims. Cl. 117-93 This invention relates to apparatus and method formaking radioactive sources. More particularly, the invention isconcerned with an improved apparatus for forming a nearly uniformdeposit of the radioactive material on a base sheet.

Conventionally, radioactive deposits are obtained by the evaporation ofan aqueous solution of salt wherein only a very minor portion of thesalt in solution is radioactive. It is desired to lay down a depositwhich is uniform as to distribution of the radioactive salts and alsouniform as to the distribution of the non-radioactive salts. Withsolutions of very high proportions of radioactive salts, the problem ofuniform distribution is increased. Accordingly, it is a primary objectof my invention to provide a technique and apparatus for producinguniform radioactive deposits from an aqueous solution. A specific objectis to provide an apparatus adapted to produce such a radioactive source.Further, an important object of my invention is to deposit a majorproportion of the total salts in solution on the base sheet. These andother objects of the invention will become apparent as the descriptionthereof proceeds.

Briefly, in accordance with this invention, a nearly uniform deposit ofradioactive material is obtained by evaporating a solution ofradioactive salt on a thin base sheet while confined within a limitedarea on the sheet. A flat sheet of material, such as mica or metal foil,is clamped against one face of a block made from teflon or othermaterial. The block contains a conical bore, the lower end of which issealed by the sheet. Thus, the sheet seals off the lower end of the borein the block and forms the bottom of a tapered cup in which the solutionof radioactive material is initially placed.

The block and sheet are mounted on an annular resilient gasket supportedon a turntable. A clamping plate is positioned over the top face of theblock and bolted at its periphery to the turntable. The clamping plateis provided with a central opening which is in register with the top ofthe cup. The turntable, and hence the cup, is rotated about its centralaxis at a constant speed.

An air jet is mounted at a small angle of discharge to the axis ofrotation and impinges on the wall of the cup at about its midpoint. Thisair jet, when so adjusted to strike the side of the cup near themeniscus surface of the liquid serves to reduce greatly the proportionof the radioactive material deposited on the sides of the cup.

This effect is due in part to the fact that the proper place-.

ment of the jet produces a wiping action on the wall surface and, inaddition, produces a stirring action which prevents salt concentrationfrom reaching saturation at the meniscus. I have found that theadjustment of the direction and strength of the air jet, so as toimpinge upon the wall at a very small angle, results in .a nearlyuniform deposit of the radioactive material over substantially only thebottom of the cup, i. e., on the base sheet. As much as 90% of theradioactive material is "ice deposited on the base sheet and only aminor portion remains on the Walls of the cup.

The rate of evaporation can be increased by adjustably I maining on thewalls of the cup can be recovered and.

utilized in the source. This is done by removing the first sheet andplacing a new or second base sheet against the bottom of the taperedbore and redissolving the wall deposit in a minimum volume of a suitablesolvent and then repeating the evaporation technique as described above.The two sheets can then be laminated with the deposits face to face toproduce a radioactive source with a minimum loss of the radioactivematerial originally placed in the cup.

For a more complete understanding of this invention and of the objectsand advantages thereof, the invention will be described in connectionwith the accompanying drawing which illustrates a preferred embodimentof the apparatus.

Referring to the drawing, I provide a drying cup assembly 10 having atop cover plate. 11 about two inches across with a central port 12 inregister with the upper end of the conical cavity 13 in the block 14. Anannular rubber gasket 15 supports the mica sheet or gold foil 16 acrossthe bottom of the conical cavity 13 in the block 14. The block 14 ispreferably formed of teflon (polytetrafiuoethylene), such materialhaving been found to facilitate the removal of the solution and depositstherefrom. The conical cavity 13 in the block 14 tapers from aboutone-half inch at the top face to about one quarter inch at the lowerface and the block 14 is about 0.375 inch thick with an overall width ofabout 1.25 inches. This gives a wall slope of about 13.

The rubber gasket 15 is provided with a central opening 17 so as toprevent the flexing of the exposed portion of the base sheet 16 when theblock 14 and the sheet 16 are clamped in place. This is done by theclamping plate 11 and the threaded pins 1% provided in the corners ofthe plate 11 and which thread into the turntable 19.

The turntable 19 is provided on its lower side with a boss 20 having anon-circular bore 21 adapted to receive the correspondingly shaped drivepin 22 of the speed reducer 23 which may be integral with the motor 24as shown in the drawing. A suitable motor 24 is a H. P. Bodine motorproducing 1800 R. P. M. with a gear reducer 23 to reduce the speed ofdrive shaft 22 to about R. P. M. The motor 24 is mounted by bolts 25a ona base 25, which provides a stable support for the entire assembly.

The drying cup assembly 10 is so arranged that the metallic foil or micasheet 16 forms the bottom of the conically shaped cavity 13 which thenbecomes the solution holder. The walls of the cavity 13 have a steeptaper sloping toward the base sheet 16 at an angle of about 5 to 15 withthe vertical axis of the cup about which it is to be rotated.

After the block 14 and sheet 16 have been mounted as illustrated, thesolution of radioactive salt is poured into place through the opening 12in the clamping plate 11 and the solution slowly evaporated to dryness.During the drying operation, the drying cup assembly 10 is iet 26 shouldbe sufiicient to form a dimple in the pool "a of liquid 2'7 in the cup13, but not sufiicient to cause the air stream to break under thesurface of the pool 27.

One suitable arrangement of the air jet 26 is illustrated in the drawingwherein the jet 26 comprises a inch 0. D. copper tubing 23 leading froma reducing valve 29. A support assembly for the jet tubing 28 comprisesa segmented rod 30-31 which is adjustably threaded into the base 25 andsecured by wing nut 32. The upper end of the rod 31 is provided with aflexible shaft 33 which supports a heat source assembly 3d including,for example, an infrared lamp 34a. keyed or indexed bayonet blade 35extending axially from the lower end of rod 31a enters the upper end ofrod 30 and permits ready removal of the jet assembly 26 and heatassembly 34 without the necessity for disturbing the adjustment of theseassemblies with respect to the cup 13 when it becomes necessary toremove the block 14 and sheet 16.

The two lengths 31 and 31a of the upper rod support are joined by twosides of a four-way threaded pipe connector 36, the two lengths oftubing 28 and 28a also being coupled by the connector 36. An air supply37 and a hose connection fitting 38 attached to the inlet end of thetubing 23a complete the assembly.

In operation, a quantity of solution containing sufficient inactive plusactive material to produce a deposit of not more than 10 mg./cm. on thebase sheet 16 is poured into the cup 13 Within the block 14 which hasbeen bolted to the turntable 19. A typical solution in cludes 3.70millicuries strontium 90 (in equilibrium with yttrium 90) chloride permilliliter of solution in weak HCl having a total solid content of 1.1milligrams per milliliter.

The air jet 26 is adjusted so as to have a terminal length of about 2inches thereof substantially parallel to the axis of the cup and so asto direct the stream of fiuid against the tapered wall of the cup 13.The air supply 37 is connected by hose connection 38 to the tubing 28aand the turntable l9 rotated at about 180 R. P. M. If heat is to be usedto accelerate the drying operation, the rod support 31 and the flexiblecable 33 are adjusted so as to place the heat source 34 in the properposition. When employing an infrared lamp of 250 watts (Westinghouse 115volt, clear lens) a spacing of about 10 to 12 inches from the lens oflamp 34a to the surface of the liquid in the pool 27 will provideadequate heat without causing the liquid 27 to boil and spatter. Theradioactive deposit upon the thin base sheet 16, resulting fromevaporation of the pool of solution 27 in the manner described, isuniform across a circular area of the sheet 16 corresponding to thebottom dimension of the conical solution holder or cup 13. A typicalbase sheet 16 upon which the salt is deposited is made of gold leaf ormica about 0.004 inch thick and suitably about 1.25 inches in diameter.

It has been found advantageous to employ an air flow at the terminal ofthe jet 26 at a rate of at least about 0.02 and below about 0.03 C. F.M. A preferred rate is 0.025 C. F. M. and such a flow provides a ratherwide path of impingement of the wall of the cavity 13 with the resultthat the air jet is continuously applied at the meniscus of the pool 27in the cavity 13. With a inch 0. D. tubing 23 and an air supply at apressure of about 3 to pounds per square inch at the reducing valve 29,a tubing length of about 2 feet gives a suitable air jet flow in the cup13.

There are several effects of the stream of air provided by jet 26. Oneis to continuously agitate the pool 27 at the meniscus-wall interface inan unsymmetrical manner. This produces a turbulence within the pool 27which prevents the formation of local super-saturated solutions whichwould result in the premature deposition of salts from the pool 27. Onthe other hand, the jet of air does not impinge as low as the junctionof the mica sheet 16 with the bottom of the sloping wall of the cavity13. Thus, there is a minor portion of salt which deposits on the loweredge 39 of the wall thereby providing a uniform coating or depositacross the entire exposed area of the sheet 16. If the air jet, on theother hand, is placed so as to displace the meniscus from the wall 39and onto an exposed annular area of the sheet 16, then in that event thedeposit is non-uniform and not reproducible.

When the salt has been evaporated to dryness by rotat ing the cup 13 ata constant speed, for example, at about 180 R. P. M. and by directingthe jet of air as described, the jet and heating assembly may betemporarily removed by separating the unit at joint 35. A new sheet 16is put in place and secured as before. A quantity of a suitable solventis then introduced into the cup 13 to dissolve the portion of the saltwhich has accumulated on the wall of the cup 13. The procedure is thenrepeated with this dilute solution of redissolved salt wherebysubstantially all of the radioactive salt is placed on one of the twosheets 16. The two sheets are then arranged with the deposits face toface and assembled in a suitable casing.

The complete source comprises a pair of thin base sheets which enclosethe radioactive deposit. The laminated unit is mounted in a split holderhaving regis tering apertures on opposite faces of the holder to providea double-faced unit. One such source is described and claimed inco-pending application S. N. 311,734 filed September 26, 1952 and issuedJanuary 18, 1955, as U. S. 2,700,111.

Relative rotary motion of the pool of liquid 27 within the cup 13 andthe stream discharged from jet 26 is essential, but can be attained byeither rotating the pool as described in some detail above or byrotating the jet. In this latter case, the jet tube 26 is crank-shapedwith the discharging end traveling within and about the cup and theinlet end being rotated through a centrally held bearing andfluid-connector means. Such bearing and fluid-connector means are wellknown and can be adapted by one skilled in the art for use with thetapered cup as described herein.

My invention has been described with reference to an apparatusspecifically designed for the fabrication of radioactive sources.However, it should be understood that this is by way of illustrationonly and that the invention is not necessarily limited thereto. It iscontemplated, therefore, that the drying cup assembly, including theturn-table, directed air jet and heat source, may be used in othersystems Where it is desired to concentrate and evaporate solutions on aminimum area. In any event, other modifications of the apparatus anduses thereof will become apparent to those skilled in the art in view ofmy description and can be made without departing from the spirit of myinvention.

What I claim is:

1. A method for preparing uniformly distributed solid deposits fromaqueous solution thereof which comprises the steps of supporting a poolof such a solution on a base sheet transparent to beta rays, rotatingsaid pool, and simultaneously directing a jet of air downwardly at theperiphery of said pool and obliquely to the top surface thereof, wherebya top-to-bottom turbulence is imparted to the pool thereby avoiding anylocalized premature supersaturation of the solution.

2. A method for preparing uniformly radioactive solid deposits from anaqueous solution of mixed radioactive strontium chloride andnon-radioactive strontium chloride which comprises the steps ofdisposing a pool of such a solution on a base sheet of mica transparentto beta rays, rotating said sheet and pool about a vertical axis atabout R. P. M., simultaneously directing a jet of air downwardly at theperipheral boundary of said pool at a rate of about 0.025 C. F. M.,whereby a topto-bottom turbulence is imparted to the pool therebyavoiding any localized premature supersaturation of the solution, andapplying radiant heat to the top surface of said pool while rotating itand while directing the said jet of air.

3. In the method of evaporatively drying a solution of salts having aminor radioactive salt composition, the improvement which includesmaintaining a pool of such solution within an open cup having slopingsides and a removable bottom, rotating said cup and pool in asubstantially horizontal plane about the axis of said cup at about 180R. P. M., downwardly jetting a stream of gasiform fluid to the peripheryof said rotating pool within said cup, the center of said streamstriking the said pool at the junction of the meniscus of the pool withthe sides of the cup, applying radiant heat to the upper surface of thepool during such rotation and jetting, and continuing the application ofheat and the jetting of the fluid until the solution is evaporated todryness.

4. In the method of evaporatively drying a solution of salts having aradioactive content, the improvement which includes maintaining a volumeof such solution in an initially deep pool of limited lateral extentwithin an open cup having sloping sides and a removable bottom, rotatingsaid cup and pool in a substantially horizontal plane about the axisthereof at about 180 R. P. M., downwardly jetting a stream of gasiformfluid to the peripheral boundary of said rotating pool, applying radiantheat to the exposed upper surface of the pool during such rotation, andcontinuing said rotating, heating and jetting until substantially all ofthe solution is evaporated to dryness on said removable bottom.

5. An apparatus for evaporating to dryness a solution of mixed solidscomprising in combination an open-ended cup comprising a tapered boreextending through a block, said bore having a taper of between about anddegrees, and a removable sheet across the smaller bottom end of the saidbore, the sloping walls of said bore and said sheet together forming acup, a turntable supporting said cup, a resilient support between saidturntable and the lower surface of said sheet, said support having anopening in register with the bottom'end of said cup, an aperturedclamping plate across the top surface of said block for removablyholding said block on said turntable with the said sheet and resilientsupport therebetween, said cup being arranged with its axis in alignmentwith the axis of rotation of said turuable, a jet tube means having itsdischarge end below the upper edge of said cup and arranged to impinge afluid stream upon the tapered wall of said cup substantially above thebottom thereof, means for supporting said jet means in adjustable arraywith respect to the said cup, a radiant heating means disposed abovesaid cup, and adjustable means for supporting said heating means.

6. An apparatus for evaporating to dryness a solution of mixed solidscomprising in combination, an open-ended cup comprising a tapered borein a block and a removable sheet across the lower end of the said bore,a turntable supporting said cup, a resilient support between saidtumtable and said sheet, an apertured clamping plate across the topsurface of said block, bolt means for removably holding said clampingplate on said turntable, the axis of said bore and of said turntablebeing in alignment, drive means for rotating said turntable, a jet tubemeans having its discharge end below the upper end of said cup andaranged to impinge a fluid stream upon the tapered bore of said cupadjacent the mid-point thereof, means for supporting said jet means inadjustable but fixed array with respect to said cup, a radiant heatingmeans disposed above said cup, and adjustable means for supporting saidheating means above said cup.

7. The apparatus of claim 6 wherein said tapered bore is in a block ofpolytetrafluorethylene and said removable sheet is transparent to betarays.

8. The apparatus of claim 7 wherein the jet tube means is arranged todischarge substantially parallel to the axis of rotation of said cup.

9. The apparatus of claim 7 wherein themeans for supporting said jet andthe adjustable means for supporting said heating means are unitary andare supported jointly by an upstanding rod having an indexed bayonetblade joint.

References Cited in the file of this patent UNITED STATES PATENTS

2. A METHOD FOR PREPARING UNIFORMLY RADIOACTIVE SOLID DEPOSITS FROM ANAQUEOUS SOLUTION OF MIXED RADIOACTIVE STRONTIUM 90 CHLORIDE ANDNON-RADIOACTIVE STRONTIUM CHLORIDE WHICH COMPRISES THE STEPS OFDISPOSING A POOL OF SUCH A SOLUTION ON A BASE SHEET OF MICA TRANSPARENTTO BETA RAYS, ROTATING SAID SHEET AND POOL ABOUT A VERTICAL AXIS ATABOUT 180 R.P.M., SIMULTANEOUSLY DIRECTING A JET OF AIR DOWNWARDLY ATTHE PERIPHERAL BOUNDARY OF SAID POOL AT A RATE OF ABOUT 0.025 C.F.M.,WHEREBY A TOPTO-BOTTOM TURBULENCE IS IMPARTED TO THE POOL THEREBYAVOIDING ANY LOCALIZED PREMATURE SUPERSATURATION OF THE SOLUTION, ANDAPPLYING RADIANT HEAT TO THE TOP SURFACE OF SAID POOL WHILE ROTATING ITAND WHILE DIRECTING THE SAID JET OF AIR.